1. Field of the Invention
The present invention relates to a magnetic core for an inductor, and more particularly, to a small-sized and low-priced magnetic core for an inductor suitable for a high-frequency signal and a large-current signal. Also, the present invention relates to a structure of a magnetic core for an inductor used in a digital audio amplifier or D-class audio amplifier.
2. Description of the Related Art
Recently, with the digitalizing progress of an electronic record signal and control signal, the digitalization of an amplifier is progressed in an audio appliance. In amplifying a digital signal of CD, MD and DVD, the digital signal has to be converted to an analog signal prior to its amplification. In this process, there are some problems in that a sound quality inferior becomes due to waveform variation caused by deterioration of a high-band signal and lack of the dynamic response characteristic, and heat is generated due to a low-efficiency amplification. In the latest digital amplifier technique, logic circuits having a good performance of converting a pulse code modulation (PCM) signal to a pulse width modulation (PWM) signal are implemented, and a high-speed and high-accuracy switching of a large-power digital signal becomes possible, thereby completely amplifying sound in a digital state. In order to be able to listen to such an amplified signal, a high-frequency carrier signal has to be removed from the amplified signal to prevent the signal from being outputted through a loudspeaker. To this end, a low pass filter having an inductor is provided in an end terminal of the amplifier.
As described above, since the signal of the digital audio amplifier is a high-frequency signal and a large-current signal, a magnetic core to be used in the digital audio amplifier should have a good saturation characteristic, and is to be constructed to shield magnetic flux due to the large-current variation not to influence peripheral circuits. Comparing to the conventional analog audio amplifier, the digital audio amplifier has a low share in a market of audio amplifier. In order to increase the market share of the digital audio amplifier, a technique of manufacturing a low-priced amplifier is required. Recently, the miniaturization of goods in household electric appliances is also an indispensable condition. Accordingly, the magnetic core in the inductor used in the digital audio amplifier has to be low-priced, a small-sized, and suitable for the large-current signal and the high-frequency signal.
The conventional magnetic core for the inductor used in the digital audio amplifier will now be described with reference to FIGS. 1A through 1C, and FIG. 2.
FIG. 1A is a perspective view of an inductor having a DR type magnetic core. FIGS. 1B and 1C are a front view of an inductor having a DR-plus-Ring type magnetic core and a perspective view of a cylindrical shield, respectively. The DR type magnetic core and the DR-plus-Ring type magnetic core are used in a power inductor suitable for a signal having a large amount of current. Since the magnetic cores are inexpensive, they are used in the digital audio amplifier. However, since the DR type magnetic core has no member for shielding the generated magnetic flux, there is a problem of completely leaking the magnetic flux. In order to solve the above problem, the DR-plus-Ring type magnetic core has been proposed. As shown in FIG. 1B, the DR-plus-Ring type magnetic core includes a cylindrical shield and the DR type magnetic core. The cylindrical shield is shown in FIG. 1C. Since the DR-plus-Ring type magnetic core is not suitable for the high frequency characteristic, like the DR type magnetic core, there is a problem that a filtered waveform has an insufficient fidelity.
The inductor having an aircore coil will now be described with reference to FIG. 2. Since the inductor does not include the magnetic core and thus is not made of magnetic material, there is little distortion of a signal waveform caused by the high-frequency characteristic of the magnetic material. Therefore, the above inductor is greatly used for the digital audio amplifier. However, since the above inductor has the increased number of windings, a capacity component and a direct current resistance component, which are generated between the windings, are increased. In addition, there are some problems that the magnetic flux is completely leaked, and since the inductor is large-sized, it is difficult to surface-mount it onto a small appliance.
That is, the conventional magnetic cores for an inductor used in the digital audio amplifier are not suitable for the large-current signal and the high-frequency signal, in addition to the problem of surface mounting.
A typical process of manufacturing a magnetic core will now be described with reference to FIG. 3. The magnetic core is made of magnetic powder. At first, the magnetic powder is manufactured through the steps of weighting, mixing, calcining, milling and spray-drying (not shown). The magnetic powder manufactured by the above process is introduced into a mold having a desired shape of magnetic core, is pressed (step 301), and is sintered (step 303) so that the magnetic core is formed. Since the magnetic core formed at the above steps has a coarse surface, an outer surface of the magnetic core is processed through a primary grinding (step 303). The primary grinding grinds the entire surface of the magnetic core, so that the coarse surface formed through the steps 301 and 302 is uniformly ground. After that, in order to form an air gap in the magnetic core, a secondary grinding is performed to grind a center portion of the magnetic core to a predetermined depth (step 304). As the secondary grinding determines a magnetic characteristic of an inductor, it requires the precision. In order to secondarily grind a plurality of magnetic cores, the step of approaching a grinder to the center portion of the magnetic core has to be performed on every magnetic core prior to the secondary grinding. Then, the grinding is performed by repeating the approaching and separating processes, with a grinding depth of the grinder being adjusted. Therefore, it is impossible to secondarily grind the plurality of magnetic cores without the approaching and the separating processes, and thus the manufacturing cost is increased.
In addition, since the magnetic powder constituting the magnetic core is relatively expensive, the purchasing cost of the magnetic powder forms a major portion of the manufacturing cost of the magnetic core. Accordingly, in order to reduce the manufacturing cost of the magnetic core for an inductor suitable for a small-sized digital audio amplifier, it is necessary to manufacture a magnetic core using a small amount of magnetic powder.